In application Ser. No. 231,151, filed Feb. 3, 1981, Allais et al., THERMAL PRINTING APPARATUS AND METHOD, which is assigned to the assignee of the present invention and which has been abandoned, a thermal printer is disclosed which is capable of providing precise printing of machine-readable characters, such as bar code characters, on a thermal print medium. Each machine-readable character typically consists of a predetermined number of sequential binary bits which, in a bar code, are represented by a sequential series of alternating bars and spaces. In one type of bar code, each bit is represented by a single bar or space, with the width of each bar and space denoting the binary value of its corresponding bit. In order to avoid code reading errors, it is very important that the width of each bar and the width of each space (sometimes referred to as the "pitch" or separation between adjacent bars) be maintained within very narrow tolerances.
A thermal printer such as that disclosed in application Ser. No. 231,151 comprises a print head assembly including a thermal print head and an opposing pressure member. The thermal print head includes at least one electrically-resistive thermal print element that is formed on a substrate, with the thermal print element being capable of producing heat upon the application of an electrical signal thereto. A thermal print medium, such as a specially-coated paper in sheet or strip form, is interposed between the thermal print head and the pressure member, whereby the pressure member maintains the thermal print medium in contact with the thermal print element. The characteristics of the thermal print medium are such that when the thermal print medium is at ambient temperature, the coating thereof is inactive. However, when the temperature of the thermal print medium is raised to or above a certain threshold temperature (which varies depending upon the type of coating and paper utilized), the coating undergoes a chemical reaction and is exposed. Such exposure results in a change in the light-reflective characteristics of the coating. In the great majority of thermal print mediums currently available, the coating is darkened upon exposure. Accordingly, when an electrical signal is applied to the thermal print element, the heat produced thereby raises the temperature of the thermal print medium above a threshold temperature so as to expose at least a portion of the coating whereby a character or a portion of a character is printed.
If a thermal printer is to be used to print machine-readable characters such as bar code characters, it is critical that the thermal printer be capable of precisely controlling the area of the thermal print medium that is exposed. The thermal printer in application Ser. No. 231,151 meets this objective in the following manner. The print head assembly includes a print head support which rigidly mounts the thermal print head in a fixed position. The print head support is composed of a material having a high thermal conductivity, and has a thermal mass that is substantially greater than the thermal mass of the thermal print head. Heating means are provided for maintaining the temperature of the print head support at a reference temperature below the threshold temperature of the thermal print medium, and a pressure member is provided that is supported in opposing relationship with the thermal print head and that is resiliently urged into contact with the thermal print element. Means are also provided for selectively transporting the thermal print medium to and from the print head assembly, the thermal print medium when so transported passing between the thermal print head and the pressure member.
The thermal print element has an area which is substantially equal to the desired incremental area of each character to be printed. In order to print such an incremental area, the thermal print medium is moved relative to the thermal print head to the position at which the incremental area is to be printed and then stopped. The mechanical arrangement described insures that the thermal print element will be substantially at the reference temperature, and that the incremental area of the thermal print medium will be urged into uniform contact with the thermal print element by the pressure member so that the incremental area of the thermal print medium will be quickly brought to the reference temperature. Thereafter, an electrical signal having a substantially constant amplitude is applied to the thermal print element for a predetermined "exposure" time that is sufficient to raise the temperature of only the incremental area of the thermal print medium in uniform contact with the thermal print element from the reference temperature to or above the threshold temperature. Upon the elapse of the exposure time, the electrical signal is immediately terminated so that only the desired incremental area of the thermal print medium is exposed. Upon the elapse of an additional time (the "rest" time) that is sufficient to allow the temperature of a thermal print element to decrease to a value that will not result in additional exposure or "smearing" of the thermal print medium upon movement thereof, the thermal print medium is moved to the position of the next incremental area to be printed and the process just described in repeated.
Although the thermal printer in application Ser. No. 231,151 is believed to be the first thermal printer including a thermal print head that is capable of printing with the very narrow tolerances required for machine-readable characters, it is subject to certain disadvantages. First, as a coded record including a plurality of successive characters is printed, the actual temperature of the thermal print element increases slightly from the reference temperature. In order to maintain the same tolerances throughout the coded record, it is necessary that the exposure and rest times be successively decreased in an empirically-determined manner from the beginning to the end of the coded record. Second, there is no provision for measuring or estimating the actual temperature of the thermal print element (or the portion of the thermal print medium in contact therewith) at any point in time, thereby necessitating empirical determinations of the exposure and rest times for each thermal print medium used. In addition, the threshold temperature will vary from sample to sample of a given thermal print medium so that it is often necessary in practice to adjust the exposure and rest times. Third, the heating means used to maintain the temperature of the print head support at the reference temperature requires a certain amount of time to bring the print head support to the reference temperature upon start-up of the thermal printer and consumes a significant amount of electrical power, thereby leading to delays in operation of the thermal printer and making the thermal printer relatively expensive to operate.
It is therefore an object of this invention to provide an improved method and apparatus for controlling the area of a thermal print medium that is exposed by a thermal printer.
It is a further object of this invention to provide such an improved method and apparatus which functions to accurately estimate the actual temperature of the thermal print element at any point in time and which accordingly eliminates the need for empirical determinations of exposure and rest times.
It is still a further object of this invention to provide such a method and apparatus which permits exposure times and rest times to be easily and quickly adjusted for variations in threshold temperature of the thermal print medium.
It is another object of this invention to provide such a method and apparatus which eliminates the need for separate heting of the print head support and which accordingly reduces start-up delays in operating the thermal printer and the cost of constructing and operating the thermal printer.